In general, in order to produce a chimeric mouse from ES cells, two types of cells, namely, an ES cell of interest and a normal mouse embryo, can be mixed to allow ontogenesis. Furthermore, ES cells are used in an attempt to produce various chimeric animals (Patent Documents 1 and 2).
Depending on the properties of the ES cells, however, ontogenesis may not proceed normally in a chimeric embryo having a high rate (chimerism rate) of mixed ES cells, in which case the embryo may die before birth. On the other hand, there are also ES cells which may result in the birth of chimeras having a high chimerism rate but those chimeras have abnormal spermatogenetic ability and thus are unable to establish a mouse strain.